1. Field of the Invention
The invention relates in general to an output buffer. More particularly, this invention relates a low noise output buffer which can simultaneously reduce switching noise and output signal ringing and maintain a DC current supply, or even with the function of reducing ground bounces.
2. Description of the Related Art
In a high operating speed digital circuit, the simultaneous switching noise (SSN) is the main source of noise. The output pad driver is the major contributor to simultaneous switching noise because of large transient currents flowing through the bounding wires, leadframe, and pin parasitic inductance. FIG. 1 shows a schematic diagram of a parasitic inductance induced after chip-package. A driver 70 having a voltage source connected to pins via pads and bonding wires induces a pin parasitic inductance 10 and a pad/bonding wire parasitic inductance 20. Similarly, at ground terminal Gnd and Load terminal C.sub.L, pin parasitic inductance 50, 30 and pad/bonding wire parasitic inductance 60, 40 are induced.
The simultaneous switching noise has the following outward effects:
(1) The maximum bouncing voltage of simultaneous switching noise between voltage source and the ground terminal (V.sub.DD /Gnd (SSN)) increases when the number of simultaneous switching output is increased. As the increase of the bouncing voltage, the time for output voltage to achieve a steady state is delayed. Consequently, the speed of the digital circuit is affected. Referring to both FIG. 2a and FIG. 2b, to the statistic analysis data, the delay time is increased when the number of switching outputs is increased. In addition, the SSN is increased as the increase of the number of the switching output. This indicates that the delay time increases when the slew rate of current increases, so as the SSN. It is known that to obtain a high speed performance, a MOS transistor of an output buffer is designed with a larger channel width to improve the capability of driving current. However, an enhanced driving current capacity induces a larger SSN and a longer delay time. It is very likely to cause a deteriorated performance or wrong function.
(2) FIG. 3 illustrates a schematic diagram of interference caused by SSN. Assuming that an active driver 90 and a quiet driver 100 have a common of V.sub.DD /Gnd, because the V.sub.DD /Gnd line are disturbed by SSN of the active driver 90, the quiet driver is disturbed through the V.sub.DD /Gnd line. When a high level H is supplied to the quiet driver 100 from an internal circuit 80, the output thereof is fixed at a low level L. Meanwhile, a signal of transferring from L to H is transmitted to n active drivers 90, so that one signal of transferring from H to L is generated by each of the n active drivers 90. n discharging currents thus flow to the internal ground terminal 95 at the same time. Since a parasitic inductance exists between the internal ground terminal 90 and an external Gnd, a spike noise is generated between internal ground terminal 95 and the external Gnd by n discharging currents (n.times.i.sub.discharge). Thus, the quiet driver 100 with an output at L is still disturbed by the spike noise through the internal ground terminal. It is possible that a receiver 110 may receive the spike noise as the signal to perform a wrong function.
FIG. 4 shows a conventional output buffer made in order to reduce SSN. The output buffer 160 includes a quiet driver 120 coupled to a quiet V.sub.DD /quiet GND (quiet V.sub.DD /GND) voltage source and a noise driver 130 coupled to a noise V.sub.DD /noise GND (noisy V.sub.DD /GND) voltage source. The quiet driver 120 further includes a quiet pull-up transistor 122, a quiet pull-down transistor 124, and a predriver composed of a first NOR gate 141 and a second NOR gate 142. The noise driver 130 includes a noise pull-up transistor 132, a noise pull-down transistor 134, and a predriver composed of a third NOR gate 143 and a fourth NOR gate 144. A first feedback NOT gate 150 and a second feedback NOT gate 152 are used to feed back a signal of an output terminal.
Under a steady state, when an input terminal 112 is H, an output terminal 114 is H, the first NOR gate 141 has L and L inputs and an H output, the quiet pull-up transistor 122 is turned on to provide H to the output terminals 114. The second NOR gate 142 has H and H inputs and an L output, the quiet pull-down transistor 124 is turned off. The third NOR gate 143 has an L input, an H input and an L output. The noise pull-up transistor 132 is turned off. The fourth NOR gate 144 has an L input, an H input and an L input. The noise pull-down transistor 132 is turned off. Meanwhile, only the quiet pull-up transistor 122 of the quiet driver 120 provides H to the output terminal.
When the input terminal enters is switched from H to L, two steps are included:
(1) Before the H state of the output terminal changes, since the input terminal 112 has been converted into L, the first NOR gate 141 has H and L inputs and L output, and the quiet pull-up transistor 122 is turned off. The inputs of the second NOR gate 142 are H and L and the output thereof is L, the quiet pull-down transistor 124 is turned off. The inputs of the third NOR gate 143 are H and H and the output thereof is L, the noise pull-up transistor 132 is turned off. The fourth NOR gate 144 has L and L inputs and H output, the noise pull-down transistor 134 is turned on to provide L to the output terminal 114. That is, a forepart of state transferring is to turn on the noise pull-down transistor 132 by the noise driver 130, so as to provide L to the output terminal 114. Meanwhile, SSN is generated in the noise GND voltage source.
(2) When the output terminal 114 is switched to L by the noise pull-down transistor 134 and fed back to the first and the second feedback NOT gates 150 and 152, the first NOR gate 141 has H and H inputs and L output, the quiet pull-up transistor 122 is thus turned off. The second NOR gate 142 has L and L inputs and an H output, so that the quiet pull-down transistor 124 is turned on to provide L to the output terminal 114. The inputs of the third NOR gate 143 are H and L and the output thereof is L, the noise pull-up transistor 132 is turned off. For the fourth NOR gate 144, the inputs are H and L and the output is H, the noise pull-down transistor 132 is turned off. That is, when the state is switched to a steady state, the quiet pull-down transistor 124 of the quiet driver 120 is turned on to provide L to the output terminal 114. Thus, SSN at the quiet GND voltage source is greatly reduced without affecting the internal circuit.
Similarly, when the input terminal 112 is switched from L to H before the output terminal 114 is converted to H, the quiet driver 120 is turned off. The noise driver 130 is turned on to bear with a large SSN at the noise V.sub.DD /noise GND voltage source. When the output terminal 114 is switched to H, the noise driver 130 is turned off, the quiet driver 120 is turned on, a smaller SSN at the quiet V.sub.DD /quiet GND voltage source is resulted.
The conventional output buffer has the following drawbacks:
(1) The output buffer uses two independent voltage sources for operation. The forepart of state transferring for the output terminal uses one voltage source, while the latter part of the state transferring uses another voltage source.
(2) When the output terminal 114 is switched from H to L, or from L to H, with regard to the first feedback NOT gate 150, a trigger level to turn off the noise driver 130 is the same as that to turn on the quiet driver 120. As a consequence, the speed of outputting signal is reduced.
(3) When the noise driver 130 is off and the quiet driver 120 are on, the slew rate to turn on the quiet pull-up or pull down transistor of the quiet driver 120 can not be too slow. However, with a very fast slew rate, SSN is increased. For the buffer 160, the SSN at quiet V.sub.DD /quiet GND is still too large.
Many attempts of fabricating an output buffer with reduced ground bounces have also been made. In U.S. Pat. No. 5,708,386 published in Jan. 13, 1998, a "CMOS output buffer with reduced L-DI/DT noise" was disclosed to achieve the objective. The patent discloses a driver circuit in which two delays are used and both turned on during a transient time to limit the time for driving an output terminal, so as to reduce the noise generation. Thus, both the delay and the transient time are fixed. Since the delay is a function of the load that the current is driving, proper operation depends on some extent of the load as known. Therefore, this type of buffer varies with different loading condition. The fixed delays thus can not work properly to reduce the noise of power line for an unknown loading.